The fusion process (e.g., downdraw process) is one of the basic techniques used to produce high quality thin glass sheets which can be used in a variety of devices like liquid crystal displays (LCDs). The fusion process is the preferred technique for producing glass sheets used in devices such as liquid crystal displays because this process produces glass sheets whose surfaces have superior flatness and smoothness when compared to glass sheets produced by other methods. The fusion process is described in U.S. Pat. Nos. 3,338,696 and 3,682,609, the contents of which are incorporated herein by reference.
The fusion process makes use of a specially shaped refractory block referred to as an isopipe (e.g., forming apparatus) over which molten glass flows down both sides and meets at the bottom to form a single glass sheet. Although the isopipe generally works well to form a glass sheet, the isopipe is long compared to its cross section and as such can creep or sag over time due to the load and to the high temperature associated with the fusion process. When the isopipe creeps or sags too much it becomes very difficult to control the quality and thickness of the glass sheet. This can be particularly troublesome when making wide glass sheets equal to or greater than Gen 10 (or 157-inch isopipe length) where it is important to ensure the uniform dimension of the glass sheet while it is being drawn in the manufacturing process. One way this problem can be addressed is by modifying the elements used to make the isopipe in a way that changes its physical properties in a direction that improves its resistance to creep.
Corning Inc. has developed several methods to improve the creep resistance of zircon which is the current material used to make the isopipe that is used to form LCD glass. These methods all relate to making a creep-resistant isopipe by mixing and homogenizing powdered zircon particles which have specific size distributions and by adding titania and yttria to the mixed zircon particles. The titania and yttria improve the grain growth of zircon by increasing the zircon grain size and decreasing the zircon grain boundary concentration in the fired isostatically pressed zircon isopipe. A detailed discussion of these different methods can be found in the following co-assigned patent applications: (1) US Patent Application No. 2008/0196449 A1; (2) US 2008/0277835 A1; (3) US 2009/0272482; and (4) PCT WO 2009/142695 A2. The contents of these documents are incorporated by reference herein. Although all of these methods work well in improving the creep resistance of the zircon isopipe there is still a desire to manufacture an improved creep-resistant zircon isopipe and this need and other needs have been satisfied by the present invention.